An electrode material used for an electrode of a vacuum interrupter (VI) etc. is required to satisfy the following characteristics: (1) high interrupting capacity; (2) high withstand voltage; (3) low contact resistance; (4) high welding resistance; (5) low contact consumption; (6) low interrupting current; (7) good workability; and (8) high mechanical strength.
Since some of the above characteristics are in a trade-off relationship, there is no electrode material satisfying all of the above characteristics. Electrode materials are thus used properly depending on the applications of interrupters, such as those for large-current interruption and for high withstand voltage. How to develop an electrode material with different characteristics has been an important issue.
In recent years, the conditions of use of vacuum interrupters have become severe, and at the same time, the range of applications of vacuum interrupters to capacitor circuits has been widening. In a capacitor circuit, a voltage twice or three times as high as the usual is applied between electrodes. On this account, it is assumed that contact surfaces of the electrodes sustain significant damage by arc generated at the time of current interruption or current switching operation, thereby easily causing the reignition of arc. There has accordingly been an increasing demand for a contact material with superior withstand voltage and current interruption capabilities to those of conventional Cu—Cr electrode materials.
As a method for production of Cu—Cr electrodes with superior electrical characteristics such as current interruption capability and withstand voltage capability, an electrode production method is known in which a Cu powder as a base material is mixed with a Cr powder for improvement of electrical characteristics and a powder of an heat resistant element (such as molybdenum (Mo), tungsten (W), niobium (Nb), tantalum (Ta), vanadium (V), zirconium (Zr) or the like) for micronization of Cr particles, followed by press-molding the mixed powder in a mold and sintering the molded body (see, for example, Patent Documents 1 and 2).
More specifically, a Cu—Cr electrode material is prepared using a Cr powder of 200 to 300 μm particle size as a raw material; and a heat resistant element is added to the Cu—Cr electrode material so as to allow micronization of the Cr powder through a microstructure technique, that is, promote alloying of Cr and the heat resistant element and enhance deposition of fine Cr—X particles (where X is the heat resistant element) in the Cu base material phase. As a consequence, the electrode has a composition in which Cr particles of 20 to 60 μM diameter are uniformly dispersed in the Cu base material phase in the form of incorporating therein the heat resistant element.
In order to improve the electrical characteristics such as current interruption capability and withstand voltage capability of the above electrode material, it is required to increase the contents of Cr and the heat resistant element in the Cu base material phase and to finely and uniformly disperse the particles of Cr and of the solid solution of Cr and the heat resistant element in the Cu base material phase.
As a result of extensive researches, the present inventors have invented an electrode material of Cu—Cr-heat resistant element (e.g. Mo) system (see, for example, Patent Documents 3 to 5). This electrode material combines uniform dispersion of fine Cr-containing particles with uniform dispersion of fine Cu structures as a highly conductive component and shows superior large-current interruption and withstand voltage capabilities.
In general, contact materials for use in interrupters etc. need to be stabilized in withstand voltage capability by a voltage-forming treatment in which fine projections or adhered foreign substances on contact surfaces are flashed over between contacts or by a current-forming treatment in which contact surfaces are melted by arc.
However, the electrode material of Cu—Cr-heat resistant element (e.g. Mo) system is higher in surface hardness and melting point than the conventional Cu—Cr electrode materials. There is thus a possibility that the energy required for stabilization of withstand voltage capability may become high. There is also a possibility that fouling caused inside the vacuum interrupter by the stabilization treatment becomes a factor of unstabilization of withstand voltage capability. Furthermore, the electrode material of Cu—Cr-heat resistant element (e.g. Mo) system is equal in energization capability to the conventional CuCr electrode materials whereby a smaller electrode diameter cannot be achieved and whereby a shortening of the time required for the forming treatment by decrease of contact area cannot be expected.